1. Field of the Invention
The present invention relates to an loss of signal detection circuit for a light receiver, which is used in optical communication or the like for detecting loss of signal in a light receiver comprising a photoelectric conversion element which performs photoelectric conversion from a light signal to an electric signal. More specifically, the present invention relates to loss of signal detection in accordance with a code error rate.
2. Description of Related Art
FIG. 9 is a block diagram of an loss of signal detector circuit for a related art light receiver 21. Referring to FIG. 9, the loss of signal detector circuit 21 comprises a photoelectric conversion element 1 for converting a light signal, which is an input, into a current signal; a transimpedance pre-amplifier 2 for converting a current signal into a voltage signal having an appropriate amplitude; a feedback resistor 3 for setting a gain of the transimpedance pre-amplifier 2; a post-amplifier 4 for amplifying an output signal of the transimpedance pre-amplifier 2; and a peak detector circuit 17 for outputting a signal having an amplification proportional to that of the output signal from the post-amplifier 4. The detector circuit 21 further comprises a threshold generating circuit 18 for generating a threshold signal for issuing an alarm signal; and a comparator 19 which outputs an alarm signal from an alarm signal output terminal 10 when the level of the output signal from the peak detector circuit 17 is higher (or lower) than that of the output signal from the threshold generating circuit 18.
The operation of the loss of signal detector circuit for light receiver 21 will be described. In the loss of signal detector circuit 21, a digital light signal, which is discriminated as a light signal xe2x80x9cHxe2x80x9d with high intensity (amplification) or as a light signal xe2x80x9cLxe2x80x9d with low intensity, is input to the photoelectric conversion element 1, which converts the input light signal into a current signal. The current signal is then input to the transimpedance preamplifier 2, which converts the current signal into a voltage signal having an appropriate amplification in accordance with the amplification of the input current signal. The voltage signal is supplied to the post-amplifier 4 where the voltage signal is amplified such that the amplification detection function can be normally operated in the peak detector circuit 17. The peak detector circuit 17 outputs a signal having an amplification proportional to the amplification of the output signal from the post-amplifier 4. The threshold generating circuit 18 generates a threshold signal at a level suitable for a particular use. The comparator 19 then compares the output signal from the peak detector circuit 17 and the threshold signal from the threshold generating circuit 18, and, when the level of the output signal of the peak detector circuit 17 is lower than that of the threshold signal from the threshold generating circuit 18, for example, outputs an alarm signal through the alarm signal output terminal 10 as an alarm output signal of the loss of signal detector circuit for a light receiver 21. In the loss of signal detector thus configured, an alarm signal can be output from the alarm signal output terminal 10 when the level of light input to the photoelectric conversion element 1 is below the predetermined level.
In recent years, there has developed a demand for light receivers capable of issuing and alarm when the code error rate of a light signal exceeds the predetermined level. However, because the code error rate largely depends on noise components of a signal, it has been impossible to use the conventional loss of signal detector circuit for a light receiver of the type described above, which relies on the intensity of light input and does not take consideration of noise components, as a circuit for detecting loss of signal in accordance with an increase in the code error rate.
The present invention was made in view of the foregoing problems of the related art, and aims to enable detection, in a light receiver, of an increase in the code error rate due to an increase in noise components of a signal caused by variations of components such as a photoelectric conversion element, a temperature change, a voltage change, or the like in such a component.
In view of the foregoing problems, in accordance with a first aspect of the present invention, there is provided a loss of signal detector circuit for a light receiver comprising a main discrimination circuit for comparing the intensity of an electric signal, obtained by photoelectrically converting a light signal using a photoelectric conversion element, with a predetermined discrimination threshold; a reference discrimination circuit for comparing the intensity of the electric signal with a reference threshold which differs from said discrimination threshold; and an operation circuit for detecting loss of the signal based on the results of the comparisons performed by said main discrimination circuit and said reference discrimination circuit.
In the above loss of signal detector circuit, said operation circuit counts, for a predetermined time period, the number of instances wherein the comparison result indicating that the intensity of an electrical signal is higher or lower than said discrimination threshold differs from the comparison result indicating that the intensity of an electrical signal is higher or lower than said reference threshold, and determines a loss of signal when the counted number exceeds a predetermined value.
Said discrimination threshold is determined such that the probability of a low level signal being judged as a high level signal equals the probability of a high level signal being judged as a low level signal when said discrimination threshold is set in said main discrimination circuit.
A light receiver comprising a photoelectric conversion element for photoelectrically converting a light signal into an electrical signal and a loss of signal detector circuit having the foregoing structure is also provided.
While normally the accuracy of discrimination and the probability of error can not be reliably determined only from the discrimination results of the main discrimination circuit, according to the structure as described above, it is possible to estimate the code error rate based on the incidence of signal misjudgment in the main discrimination circuit from the discrimination results with regard to both the discrimination threshold and the reference threshold which differs from the discrimination threshold.
In accordance with a second aspect of the present invention, there is provided a loss of signal detector circuit for a light receiver comprising a main discrimination circuit for comparing the intensity of an electric signal, obtained by photoelectrically converting a light signal using a photoelectric conversion element, with a predetermined discrimination threshold at a predetermined discrimination timing; a reference discrimination circuit for comparing the intensity of the electric signal with said discrimination threshold at a reference timing other than said discrimination timing; and an operation circuit for detecting loss of the signal based on the results of the comparisons performed by said main discrimination circuit and said reference discrimination circuit.
Said operation circuit counts, for a predetermined period of time, the number of instances wherein the comparison result indicating that the intensity of an electrical signal is higher or lower said discrimination threshold at said discrimination timing differs from the comparison result indicating that the intensity of an electrical signal is higher or lower than said discrimination threshold at said reference timing, and determines a loss of signal when the counted number exceeds a predetermined value.
Said discrimination timing is determined such that the probability that an electrical signal which should reach the discrimination threshold at an earlier signal change timing in an eye pattern actually reaches the discrimination threshold at a timing later than said earlier signal change timing equals the probability that an electrical signal which should reach the discrimination threshold at a later signal change timing in the eye pattern actually reaches the discrimination threshold at a timing earlier than said later signal change timing, when said discrimination timing is set in said main discrimination circuit.
A light receiver comprising a photoelectric conversion element for photoelectrically converting a light signal into an electric signal and a loss of signal detector circuit having the foregoing structure is also provided.
With the above structure, it is possible to detect the code error rate in the main discrimination circuit from the discrimination results at the discrimination timing and the discrimination results at the reference timing, which differs from the discrimination timing.
Further, in accordance with a third aspect of the present invention, there is provided a loss of signal detector circuit for a light receiver comprising a main discrimination circuit for comparing the intensity of an electric signal, obtained by photoelectrically converting a light signal using a photoelectric conversion element, with a predetermined main discrimination threshold at a predetermined main discrimination timing; a first reference discrimination circuit for comparing, at said main discrimination timing, the intensity of the electric signal with a reference threshold other than said main discrimination threshold; a second reference discrimination circuit for comparing the intensity of the electric signal with said main discrimination threshold at a reference timing other than said main discrimination timing; and an operation circuit for detecting loss of the signal based on the results of the comparisons performed by said main discrimination circuit, said first reference discrimination circuit, and said second reference discrimination circuit.
A light receiver comprising a photoelectric conversion element for photoelectrically converting a light signal into an electric signal, and a loss of signal detector circuit having the foregoing structure is also provided.
With the above structure, it is possible to detect the code error rate in the main discrimination circuit based on the comparison results with regard to the discrimination threshold at the discrimination timing, the comparison results with reference to the reference threshold which differs from the discrimination threshold, and the comparison results at the reference timing which differs from the discrimination timing.